The present invention generally relates to a method for fabricating a tunable, 3-dimensional solenoid incorporating an inductor coil and an inductance control means and device fabricated, and more particularly, relates to a method for fabricating a tunable, 3-dimensional solenoid by a micro-electro mechanical system (MEMS) wherein a tunable, or variable 3-D inductor coil is fabricated by CMOS technology and device fabricated by the method.
Miniaturization of motors, actuators and similar machine parts is receiving increasing attention because of the new uses of these devices made possible because of their small size. Additionally, these devices can be manufactured in large quantities at low piece-part cost. Current designs of miniaturized machine parts can be categorized according to size or scale. Macroscopic machine parts have a length in the range of approximately 1 to 10 inches, and while microscopic machine parts, sometimes referred to as MEMS (Micro Electro Mechanical Systems) have a length in the range of 0.01 to 1 inch.
In any event, existing miniaturized actuators and motors of both macroscopic and microscopic size are essentially replicas of larger motors, and thus include such component parts as windings, stators, gears, transmission links, etc. These miniaturized parts must be assembled with high precision in order to produce an operable device providing the desired function, e.g. movement of an electrically activated component that then mechanically engages other parts to induce motion. Depending upon the engagement configuration, this motion may be linear in any of several axes, rotary, circular, etc. Because of the number of complex parts that must be assembled with a high degree of precision, the yields of parts meeting target specifications and performance are relatively low using current manufacturing processes. These low yields in turn increase the cost of the parts. Accordingly, it would be desirable to provide a new form of actuator and related method for inducing movement of an object on a microscopic or macroscopic scale which eliminates the problems mentioned above.
The MEMS technology has recently been extended to the semiconductor fabrication industry. In the present state of the art, a semiconductor device is normally formed in a planar structure and therefore the process for fabricating the semiconductor device is generally a planar process. For instance, layers of different materials, i.e. such as insulating materials and metallic conducting materials, are deposited one on top of one another and then features of the device are etched through the various layers. The planar fabrication process, while adequate in fabricating most semiconductor elements and devices, is not suitable for fabricating certain devices that are 3-dimensional in nature. For instance, a 3-D solenoid, i.e. or a 3-D inductor coil, must be fabricated by stacking a large number of layers from the bottom to the top and therefore, requires a large number of photomasks to complete the task. For instance, when CMOS technology is used in forming such 3-D solenoid, at least four other steps utilizing photomasks must be incorporated in order to complete the fabrication process. Moreover, the precise alignment between the layers is necessary in order to avoid a variety of processing difficulties occurring at the interfaces.
Another limitation imposed by the planar processing technology is that only a square or rectangular-shaped 3-D solenoid can be fabricated. A 3-D solenoid of circular shape cannot be fabricated by such technology. In order to raise a 3-D solenoid from a semiconductor substrate, very thick photoresist layers and electroplating techniques for filling large aspect ratio structures must also be utilized, which further increases the complexity of the fabrication process.
3-D solenoids or inductor coils have been widely used in radio frequency (RF) communication technologies. It is especially critical for RF passive telecommunication devices which require high quality factor inductors. For instance, such high quality factor inductors include those utilized in RF filters or RF oscillators. Presently, RF telecommunication devices utilize inductor coils that are planar inductor coils which produces a magnetic field that is perpendicular co the device substrate. As a result, induced currents are produced in a silicon substrate and induced, thus causing significant energy loss, and consequently, leading to a low quality factor. This prevents the use of such devices at even higher radio frequencies. For instance, presently fabricated components for telecommunication equipment such as passive elements of inductor coils, capacitors and resistors cannot be fabricated on the same silicon substrate with the active elements. Instead, such passive elements are assembled together with the active elements on a circuit board producing a circuit board of very large area to accommodate the passive elements. If the passive elements can be combined with the active elements on the same semiconductor substrate, the size of the communication module can be significantly reduced.
It is therefore an object of the present invention to provide a method for fabricating a tunable, 3-D solenoid that does not have the drawbacks or shortcomings of the conventional methods for fabrication.
It is another object of the present invention to provide a tunable, 3-D solenoid which can be fabricated by a MEMS technology.
It is a further object of the present invention to provide a tunable, 3-D solenoid that can be fabricated on a semiconductor substrate by CMOS technology.
It is another further object of the present invention to provide a tunable, 3-D solenoid fabricated by MEMS technology such that the solenoid can be self-assembled without the need of any additional actuation or monitoring.
It is still another object of the present invention to provide a method for fabricating a tunable, 3-D solenoid by a CMOS technology on a silicon substrate and then raising the planar spiral of the inductor from the substrate and pulling apart into a 3-D coil.
It is yet another object of the present invention to provide a method for fabricating a tunable, 3-D solenoid monolithically on one chip and then raising the planar spiral fabricated by differences in CTE""s or residual stress.
In accordance with the present invention, a method for fabricating a tunable, 3-dimensional solenoid and the solenoid device fabricated are disclosed.
In a preferred embodiment, a method for fabricating a tunable, 3-dimensional solenoid can be carried out by the operating steps of providing a pre-processed semiconductor substrate; depositing a first silicon dioxide layer on a top surface of the substrate; depositing a layer of a first metal on the first silicon dioxide layer; patterning the first metal layer into two lower electrodes; depositing a second silicon dioxide layer on top of the two lower electrodes and the first silicon dioxide layer; depositing a second metal layer on top of said second silicon dioxide layer; patterning said second metal layer into an inductor coil having a first end at near the center and a second end at an outer periphery of said inductor coil; depositing a third silicon dioxide layer on top of said inductor coil and said second silicon dioxide layer; patterning via openings in the third silicon dioxide layer exposing the first end and the second end of the inductor coil; filling the via openings with a third metal forming two vias over the first end and the second end of the inductor coil, respectively; depositing a fourth silicon dioxide layer over the two vias and the third silicon dioxide layer; patterning first trench openings for curved arms in the fourth silicon dioxide layer with inner ends of the first trench openings exposing the two vias; depositing a layer of the third metal into the trenches forming two curved arms; depositing a fifth silicon dioxide layer over the two curved arms and the fourth silicon dioxide layer; patterning second trench openings on and exposing the two curved arms; depositing a fourth metal in the second trench openings, the fourth metal may have a coefficient of thermal expansion smaller, or residual stress larger, than that for the third metal forming bi-layered curved arms; and removing the third, fourth and fifth silicon dioxide layers from the top surface of the substrate and exposing the inductor coil, the vias and the bi-layered curved arms.
The method for fabricating a tunable 3-dimensional solenoid may further include the step of filling the via openings with W, or the step of filling the via openings by a W CVD process. The method may further include the step of depositing the first and second and third metal layers of AlCu and the fourth metal of Ni forming the bi-layered curved arms. The method may further include the step of depositing the first, second and third metal layers of AlCu and the fourth metal of Cr forming the bi-layered curved arms. The first and second metal layers forming the two lower electrodes and the inductor coil may be AlCu. The method may further include the step of removing the third, fourth and fifth silicon dioxide layers by a wet etch technique or by a reactive ion etching technique. The method may further include the step of depositing the fourth metal of Ni by an electroless plating technique, or the step of depositing the fourth metal of Cr by a sputtering technique.
The present invention is further directed to a tunable, 3-dimensional solenoid which includes a pre-processed semiconductor substrate having a silicon dioxide layer on top; two spaced-apart lower electrodes embedded in said silicon dioxide layer; two curved arms each having a free end spaced-apart from a top surface of the silicon dioxide layer and a fixed end attached to the top surface of the silicon dioxide layer, each of the two curved arms may be formed of a bi-layer metal laminate wherein a bottom metal layer is formed of a first metal and a top metal layer is formed of a second metal that has a coefficient of thermal expansion smaller, or residual stress larger, than that of the first metal such that the ends of the curved arms curve upwardly away from the substrate; an inductor coil formed of a conductive metal that has two ends each connected to one of the free ends of the curved arms, respectively, for providing electrical communication between the two fixed ends of the curved arms; and an inductance control means for feeding a voltage to said two lower electrodes and said two curved arms to control a deflection of the two curved arms away from the substrate.
In the tunable, 3-dimensional solenoid, the substrate may be a Si substrate, the first metal forming the bottom metal layer may be AlCu, the second metal forming the top metal layer may be Ni or Cr, and the two lower electrodes and the inductor coil are formed of AlCu. The tunable, 3-dimensional solenoid may further comprising via means connecting in-between the free ends of the curved arms and the two ends of the inductor coil, respectively. The solenoid may be formed in a ground-signal-ground configuration for RF communication. The inductor coil may be formed in circular shape or formed in rectangular shape. The via means may be formed of a metal.